Mammography method and apparatus

ABSTRACT

A system for generating a three-dimensional image of the compressed breast  40  of a subject includes an x-ray mammography unit  24  for generating x-ray mammography data, a mechanical scanner  20  including an x-ray mammography compression paddle assembly  22 , a control and motion system  26, 28  for driving the mechanical scanner  20  and for sensing the control and motion system&#39;s position, an ultrasound probe  32  for generating ultrasound image data in spatial registration with the x-ray mammography unit  24 , and a computer  38  for generating from the ultrasound image data and the x-ray mammography data the three-dimensional ultrasound image. A method of examining a breast of a subject includes contacting an anterior surface of the breast with a compression paddle, applying pressure to the anterior surface of the breast with the compression paddle to compress it to reduce the thickness of the breast tissue, passing an ultrasound beam having a frequency greater than 3 MHz, preferably about 5 MHZ or more, through the paddle and the compressed breast tissue, receiving echoes from the compressed breast tissue through the compression paddle, and converting the echoes into breast examination data.

RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.09/448,675, filed Nov. 24, 1999, now U.S. Pat. No. 6,574,499, which isbased on U.S. provisional application Ser. No. 60/109,991, filed Nov.25, 1998, the disclosures of which are hereby incorporated herein byreference.

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable tams as provided for by the terms of NIH GrantNumber 2 R44 CA65225 awarded by the National Institutes of Health.

FIELD OF THE INVENTION

This invention relates to mammography methods and to apparatus forcarrying out such methods.

BACKGROUND OF THE INVENTION

Currently, on an international scale, ultrasound breast examination isan accepted medical modality applied both as a primary method forevaluation of the breasts of young patients, that is, those under 40years of age, and as an adjunct to x-ray mammography. See, for example,Kasumi, F., et al., “Topics in Breast Ultrasound,” Seventh InternationalCongress on the Ultrasonic Examination of the Breast, ShinoharaPublications, Inc., 1-7, Hongo 2-chome, Bunkyo-ku, Tokyo 113, Japan,1991; Tohno, E., et al., Ultrasound Diagnosis of Breast Diseases, NewYork, Churchill-Livingstone, 1994; and, Stavros, A. T., et al., “SolidBreast Nodules: Use of Sonography to Distinguish Between Benign andMalignant Lesions,” Radiology 196, pp. 123-134, 1995. In terms of thediagnostic effectiveness of the ultrasound breast imaging, a number ofinvestigators from the early 1980s to the present have shown that thismodality is not limited to diagnosing the solid or cystic nature of abreast mass. It is capable, with a high degree of accuracy, of providingimaging data which permits differentiation of benign and malignantbreast masses. See, for example, Stavros, A. T., et al., “Solid BreastNodules: Use of Sonography to Distinguish Between Benign and MalignantLesions,” Radiology 196, pp. 123-134, 1995; Kelly-Fry, E., et al.,“Factors Critical to Highly Accurate Diagnosis of Malignant BreastPathologies by Ultrasound Imaging,” Ultrasound 82, eds., Lerski, R. A.,et al., Pergamon Press, Oxford and New York, 1983; Harper, P., et al.,“Breast Ultrasound: Report of a 5-Year Combined Clinical and ResearchProgram,” Le Journal Francais d'Echographie, 2n 5, pp. 133-139, 1984;Ueno, E., et al., “Classification and Diagnostic Criteria in BreastEchography,” Japan Journal of Medicine, Ultrasonics, vol. 13, no. 1, pp.19-31, 1986 (in English); Ueno, E., et al., “Dynamic Tests in Real-TimeBreast Echography,” Ultrasound in Med. & Biology, 14 (supp. 1), pp.53-57, 1988; Tohnosu, N., et al., “Clinical Evaluation of Ultrasound inBreast Cancers in Comparison with Mammography, Computed Tomography andDigital Subtraction Angiography,” Topics in Breast Ultrasound, eds.,Kasumi, F., et al., Shinohara Pub. Inc., Tokyo, Japan, 1991; and,Gerlach, B., et al., “Comparison of X-ray Mammography andSonomammography of 1,209 Histological Verified Breast Diseases,” BreastUltrasound Update, eds., Madjar, H., et al., Karger, Basel, Freiburg,N.Y., 1994. In Japan, ultrasound breast imaging has equal diagnosticstatus with x-ray mammography. See, for example, Ueno, E., et al.,“Classification and Diagnostic Criteria in Breast Echography,” JapanJournal of Medicine, Ultrasonics, vol. 13, no. 1, pp. 19-31, 1986 (inEnglish); Ueno, E., et al., “Dynamic Tests in Real-Time BreastEchography,” Ultrasound in Med. & Biology, 14 (supp. 1), pp. 53-57,1988; Tohnosu, N., et al., “Clinical Evaluation of Ultrasound in BreastCancers in Comparison with Mammography, Computed Tomography and DigitalSubtraction Angiography,” Topics in Breast Ultrasound, eds., Kasumi, F.,et al., Shinohara Pub. Inc., Tokyo, Japan, 1991. European investigatorshave found that ultrasound breast imaging can equal the accuracy ofx-ray mammography in the diagnosis of overt, malignant breast masses.See, for example, Gerlach, B., et al., “Comparison of X-ray Mammographyand Sonomammography of 1,209 Histologically Verified Breast Diseases,”Breast Ultrasound Update, eds., Madjar, H., et al., Karger, Basel,Freiburg, N.Y., 1994; Dambrosio, F., et al., “Clinical Program of BreastSurveillance by Means of Echopalpation: Results from January 1985 to May1992,” Breast Ultrasound Update, eds., Madjar, H., et al., Karger,Basel, Freiburg, N.Y., 1994; and, Leucht, W., et al., “Is BreastSonography an Additional Method for the Diagnosis of Palpable Masses,”Topics in Breast Ultrasound, eds., Kasumi, F., et al., Shinohara Pub.Inc., 11-7 Hongo 2-chome, Bunkyo-ku, Tokyo 113, Japan, 1991. In theUnited States, many clinicians during the 1980s and early 1990srestricted ultrasound breast imaging to a limited role ofdifferentiation between cystic and solid masses. See, for example,Sickles, E. A., “Imaging Techniques Other Than Mammography for theDetection and Diagnosis of Breast Cancer,” Recent Results in CancerResearch, 119, pp. 127-135, 1990; Jackson, V. P., “The Role of US inBreast Imaging,” Radiology, 177, pp. 305-311, 1990; Bassett, L. W., etal., “Breast Sonography,” American Journal of Radiology, 156 (3), pp.449-455, 1991; Feig, S. A., “Breast Masses: Mammographic and SonographicEvaluation,” Radiol. Clin. North Am., 30, pp. 67-93, 1992; and, Orel, S.G., et al., “Nonmammographic Imaging of the Breast: Current Issues andFuture Prospects,” Sem. In Roentgenology, XXVIII, no. 3, pp. 231-241,1993. Following the 1995 publication of a clinical study which providedfurther data on the successful differentiation of benign and malignantmasses by ultrasound breast imaging techniques, this modality was morewidely applied in the United States. See, for example, Stavros, A. T.,et al., “Solid Breast Nodules: Use of Sonography to Distinguish BetweenBenign and Malignant Lesions,” Radiology 196, pp. 123-134, 1995.

Since the 1980s, most ultrasound breast examinations have been carriedout with the patient in the supine position. Imaging is carried out bymoving a handheld ultrasound transducer across the free flowing surfaceof the breast and recording the images on film. By contrast, for x-raymammography, the patient is in a standing or sitting position with thebreast compressed between a plastic paddle and the surface of an x-rayfilm holder module. The breast is alternately compressed in variousorientations such as cranio-caudal, lateral and oblique while the x-raybeam traverses the breast in each of these positions. For eachindividual position, an image is recorded. Currently, to correlateprecisely standard breast ultrasound imaging data with that provided byx-ray mammography data can sometimes be impossible because theanatomical orientations of tissues traversed by the x-ray beam for thevarious compressed breast positions are different from the anatomicalposition of tissues traversed by the ultrasound beam following itsentrance into an uncompressed breast in a supine position. Also, sincetissue is mobile, the location of a breast mass as imaged when a breastis compressed between two plates can be different from that of itsimaged location when the breast is uncompressed and in a supineposition. These problems can lead to diagnostic errors.

In an attempt to improve correlation between ultrasound and x-rayimaging data, in 1983 Novak demonstrated a technique for holding thebreast in the same positions used in x-ray mammography while applying alinear array ultrasound transducer in direct contact with the breastsurface. See, Novak, D., “Indications for and Comparative DiagnosticValue of Combined Ultrasound and X-ray Mammography,” European Journal ofRadiology, 3, 1983. A plexiglas plate was used as a support on one sideof the breast while the ultrasound transducer contacted the skin surfaceof the opposite side. The breast was not compressed between two plates.

In the early 1990s, Kelly-Fry, et al., demonstrated that speciallydesigned breast compression paddles, constructed from various types andthicknesses of plastics, including polyesters, polycarbonates andacrylics, can transfer both x-ray and ultrasound, without seriousattenuation of either modality. See, for example: Kelly-Fry, E., et al.,“A New Ultrasound Mammography Technique That Provides ImprovedCorrelation With X-ray Mammography,” Amer. Col. Radiol., 24^(th)National Conference on Breast Cancer, New Orleans, La., March 1990;Kelly-Fry, E., et al., “Adaptation, Development and Expansion of X-rayMammography Techniques for Ultrasound Mammography,” Journal ofUltrasound in Medicine, 10, no. 3, S 16, supplement, March 1991;Kelly-Fry, E., “New Techniques for Ultrasound Mammography,” NationalCancer Institute Breast Imaging Workshop, Bethesda, Md., Sep. 4-6, 1991;and, Kelly-Fry, E., et al., “Rapid Ultrasound Scanning of Both BreastsPositioned and Compressed in the Mode of X-ray Mammography,” Journal ofUltrasound in Medicine, 13, no. 3, S41-42 supplement, March, 1994.Instrumentation systems which incorporated these compression paddleswere designed and applied to patients with the purpose of ultrasonicallyimaging a breast while it was held under the same compression andposition orientations used in x-ray mammography. A hand-held ultrasoundlinear array transducer placed in contact with the compression plate wasused for imaging.

Subsequent investigations of this approach by Dines, et al.,Romilly-Harper, et al., and Kelly-Fry, et al., included automation ofthe transducer motion, use of high ultrasound frequencies, such as, forexample, 7.5 MHz, 10 MHz and 13 MHz, 3D ultrasound imaging and clinicalapplication of the system. See, for example, Dines, K. A., et al.,“Automated Three-Dimensional Ultrasound Breast Scanning in theCraniocaudal Mammography Position,” Ninth International Congress on theUltrasonic Examination of the Breast, Sep. 28-Oct. 1, 1995, pp. 43-44;Dines, K. A., et al., “Automated Three-Dimensional Ultrasonic BreastScanning in the Compressed Mammography Position,” Journal of Ultrasoundin Medicine, vol. 18, no. 3, supplement, March, 1999; Romilly-Harper, A.P., et al., “Clinical Evaluation of Manual, Automated and 3-D UltrasoundImaging of Breasts Compressed in the Same Position Modes Applied inX-ray Mammography,” Ninth International Congress on the UltrasonicExamination of the Breast, Sep. 28-Oct. 1, 1995, pp. 45-46; and,Kelly-Fry, E., et al., “Mammography Instrumentation for Combined X-rayand Ultrasound Imaging,” Ninth International Congress on the UltrasonicExamination of the Breast, Sep. 28-Oct. 1, 1995.

To obtain data on the ultrasound attenuation and velocity of breasttumors, Richter designed a system in which a breast is compressedbetween two thick, for example, approximately 0.39 inch (10 mm),plexiglas plates, in the craniocaudal position. A metal reflector isplaced on the inferior plexiglas plate and a linear array transducer isin contact with the upper plate. See, for example, Richter, K.,“Technique for Detecting and Evaluating Breast Lesions,” Journal ofUltrasound in Medicine, 13, pp. 797-802, 1994 and Richter, K. “Detectionof Diffuse Breast Cancers with a New Sonographic Method,” J. Clin.Ultrasound, 24, pp. 157-168, May, 1996. No x-ray imaging system wasincluded in this initial instrumentation. The thick plexiglascompression paddle was inappropriate for x-ray breast imaging because ofits increased attenuation of the x-ray beam. See, for example,Kelly-Fry, E., et al., “Mammography Instrumentation for Combined X-rayand Ultrasound Imaging,” Ninth International Congress on the UltrasonicExamination of the Breast, Sep. 28-Oct. 1, 1995. The thickness of thecompression plate was also inappropriate for ultrasound imaging, causingincreased ultrasound attenuation and multiple artifactual reflectionswithin the breast image. In subsequent investigations, Richter, et al.,carried out clinical studies at a low frequency, 5 MHz, using automatedtransducer motion with attachment of the imaging system to a standardx-ray unit. See, for example, Richter, et al., “Description and FirstClinical Use of a New System for Combined Mammography and AutomatedClinical Amplitude/Velocity Reconstructive Imaging (CARI) BreastSonography, Invest. Radiol., 32, pp. 19-28, 1997, Richter, K., et al.,“Detection of Malignant and Benign Breast Lesions with an Automated USSystem: Results in 120 Cases,” Radiology, 205, pp. 823-830, 1997;5,603,326; and, 5,840,022.

Other patents illustrate and describe instrument systems which combinex-ray mammography and ultrasound mammography using breast compressionmaterials that are radiolucent and sonolucent. See, for example, U.S.Pat. Nos. 5,474,072; 5,479,927; and WO 95/11627. Earlier publications onthe development and application of a combined x-ray and ultrasoundmammography system using breast compression paddles that transmit bothx-rays and ultrasound are not referenced. A breast examination systembased upon these references was commercially marketed as a 3Dultrasound-guided breast biopsy system.

U.S. Pat. No. 5,776,062 illustrates and describes a system for applyingx-rays to identify a region in a breast containing a possible malignantmass. Subsequently, ultrasound imaging is performed in order to targetthe x-ray identified region. Ultrasound-guided biopsy is then based onthe combined data. The system is not designed for ultrasound scanning ofthe whole breast. Ultrasound imaging takes place via an opening in asubstitute breast compression paddle, rather than via application of anultrasound transducer in direct contact with the breast compressionpaddle used for the x-ray imaging. Interruption between the x-ray andultrasound imaging procedures is required for this procedure.

With respect to 3-D ultrasound imaging, Itoh, et al., developed an earlyultrasound instrumentation system which provided just the outlines, thatis, the shape, in three dimensions, of a breast mass. See, for example,Itoh, et al., “A Computer-Aided Three-Dimensional Display System forUltrasonic Diagnosis of a Breast Tumor,” Ultrasonics, pp. 261-268,November, 1979. The 3-D images only included breast tumor contouroutlines obtained by digitizing and computer processing image data fromstandard B-mode volume scans.

In 1982, J. F. Greenleaf carried out investigations of 3-D ultrasoundimaging of excised breasts by digitizing and computer processingstandard B-mode image data. See Greenleaf, J. F., “Three-DimensionalImaging in Ultrasound,” J. of Med. Systems, vol. 6, no. 6, pp. 580-589,1982.

Rotten, et al., performed 3-D breast imaging using direct contact of astandard ultrasound transducer on the uncompressed breasts of subjectslying in supine position. See, for example, Rotten, D., et al., “ThreeDimensional Imaging of Solid Breast Tumors With Ultrasound: PreliminaryData and Analysis of Its Possible Contribution to the Understanding ofthe Standard Two-Dimensional Sonographic Images,” Ultrasound Obstet.Gynecol., vol. 1, pp. 384-390, 1991, and Rotten, D., et al., “Analysisof Normal Breast Tissue and of Solid Breast Masses UsingThree-Dimensional Ultrasound Mammography,” Ultrasound Obstet. Gynecol.,vol. 14, 114-124, 1999. This image data was processed by a graphic workstation with three-dimensional software. The system was not designed fora precise comparison between ultrasound images and x-ray images in termsof ultrasonically imaging a breast while it is held in the samepositions and under the same compression for each modality.

Hernandez, et al., in an investigation of stereoscopic visualization of3D ultrasound breast images used a plexiglas plate to compress a breastin a craniocaudal position. A linear phased array transducer wasautomatically translated across the compressed breast. The ultrasoundimaging was not performed by directing the ultrasound through theplexiglas, but rather, by directing the ultrasound through an opening inthe plexiglas. See Hernandez, A., et al., “Stereoscopic Visualization ofThree-Dimensional Ultrasonic Data Applied to Breast Tumors,” Eur. J.Ultrasound, vol. 8, no. 1, pp. 51-65, September 1998.

Various other apparatus and methods for conducting mammography areknown. There are, for example, the methods and apparatus described inthe following listed references: 5,640,956; 5,664,573; 5,938,613;Kelly-Fry, E., et al., “The Rationale For Ultrasound Imaging of BreastsCompressed and Positioned in the Modes Applied in X-ray Mammography,”International Breast Ultrasound School, Sep. 28-Oct. 1, 1995, pp.126-129. This background is not intended as a representation that athorough search of the prior art has been conducted or that no morepertinent art than that listed above exists, and no such representationshould be inferred.

Though x-ray mammography is a well-accepted imaging modality for breastcancer detection, it has several shortcomings. First of all, only athrough-transmission image related to integrated tissue density isobtained. Overlying diagnostic features are summed together, resultingin the possibility that important information is blurred, summed, andoverlaid so it cannot be detected in the x-ray image. A furthershortcoming is that the breast is imaged only up to the chest wall, butthere may be abnormalities further in that are not recorded on the x-rayfilm. The present invention provides an additional imaging viewparticularly appropriate for this latter situation.

DISCLOSURE OF THE INVENTION

According to one aspect of the invention, a system for generating athree-dimensional image of the compressed breast of a subject includesan x-ray mammography unit for generating x-ray mammography data, amechanical scanner including an x-ray mammography compression paddleassembly, a control and motion system for driving the mechanical scannerand for sensing the control and motion system's position, an ultrasoundprobe for generating ultrasound image data in spatial registration withthe x-ray mammography unit, and a computer for generating from theultrasound image data and the x-ray mammography data thethree-dimensional ultrasound image.

Illustratively according to this aspect of the invention, the ultrasoundprobe is a linear array probe.

Further illustratively according to this aspect of the invention, theapparatus includes a display for displaying the three-dimensionalimages.

Additionally illustratively according to this aspect of the invention,the apparatus includes a display for displaying two-dimensionalultrasound images.

Illustratively according to this aspect of the invention, thetwo-dimensional ultrasound images include B-mode images and the displayfor displaying two-dimensional ultrasound images is a display fordisplaying B-mode images.

Additionally illustratively according to this aspect of the invention,the apparatus includes an image capture device for capturing the B-modeimages.

Illustratively according to this aspect of the invention, the x-raymammography unit includes a vertical x-ray support column with a movablearm. The arm supports an x-ray tube. The x-ray mammography unit furtherincludes a movable paddle mount block, a detector assembly including anx-ray image detector, and a lower compression surface for supporting theunderside of the compressed breast.

Further illustratively according to this aspect of the invention, themechanical scanner is connected to the compression paddle assembly. Thecompression paddle assembly is connected to the paddle mount block.Force applied by the paddle mount block compresses the breast under thecompression paddle assembly.

Additionally illustratively according to this aspect of the invention,the movable arm can be rotated in a plane parallel to the patient'schest wall and positioned vertically so that the patient's breast can beinserted between the compression paddle assembly and the lowercompression plate over a range of angular rotations of the movable arm.

Illustratively according to this aspect of the invention, the paddlemount block is mounted so as to permit it to be translated far enough toprovide enough room for the patient's breast to fit between thecompression paddle assembly and the detector assembly, for both thecranio-caudal and lateral-oblique positions. Further, such translationis designed to permit the patient's body to fit between the compressionpaddle assembly and the detector assembly for imaging in the head-onposition.

Further illustratively according to this aspect of the invention, themotion system includes a three-dimensional mechanical positioning systemfor scanning an ultrasound probe across the breast under control of thecomputer to yield a three-dimensional image registered to a spatialcoordinate frame.

Additionally illustratively according to this aspect of the invention,the mechanical scanner includes at least one X-axis actuator, at leastone Y-axis actuator, and a Z-axis positioner. Illustratively, at leastone Y-axis actuator includes a left Y-axis actuator and a right Y-axisactuator. Further illustratively, the left Y-axis actuator and the rightY-axis actuator are attached in parallel to a support bar.

Illustratively according to this aspect of the invention, the X-axisactuator is mounted across the Y-axis actuators for positioning theZ-axis positioner carrying the ultrasound probe.

Further illustratively according to this aspect of the invention, thecompression paddle assembly is fitted between the Y-axis actuators.

Additionally illustratively according to this aspect of the invention,the compression paddle assembly includes an ultrasound imagingcompression paddle constructed from plastic. Illustratively, the plasticis a polycarbonate plastic.

Further illustratively according to this aspect of the invention, theapparatus includes a Z-axis position encoder for providing an input tothe computer for use in three-dimensional image construction.

Illustratively according to this aspect of the invention, the Z-axisposition encoder includes an encoder linkage, a linear encoder sensor,and a linear encoder graticule for monitoring the Z-position of theultrasound probe.

According to another aspect of the invention, a method of examining abreast of a subject includes contacting an anterior surface of thebreast with a compression paddle, applying pressure to the anteriorsurface of the breast with the compression paddle to compress it toreduce the thickness of the breast tissue between the compression paddleand the anterior wall of the subject's chest, passing an ultrasound beamhaving a frequency of greater than 3 MHz, preferably about 5 MHz ormore, through the paddle and the compressed breast tissue, receivingechoes from the compressed breast tissue through the compression paddle,and converting the echoes into breast examination data.

Illustratively according to this aspect of the invention, contacting ananterior surface of the breast of the subject includes contacting ananterior surface of the breast of a standing subject.

Alternatively illustratively according to this aspect of the invention,contacting an anterior surface of the breast of the subject includescontacting an anterior surface of the breast of a sitting subject.

Further illustratively according to this aspect of the invention,contacting an anterior surface of the breast with a compression paddleincludes contacting an anterior surface of the breast with a compressionpaddle constructed from plastic.

Additionally illustratively according to this aspect of the invention,contacting an anterior surface of the breast with a compression paddleconstructed from plastic includes contacting an anterior surface of thebreast with a compression paddle constructed from a polycarbonateplastic.

Illustratively according to this aspect of the invention, contacting ananterior surface of the breast with a compression paddle includescontacting an anterior surface of the breast with a compression paddlehaving a thickness not greater than about 0.12 inch (about 3 mm).

Further illustratively according to this aspect of the invention,applying pressure to the anterior surface of the breast with thecompression paddle to compress it includes applying pressure to theanterior surface of the breast with the compression paddle under motorcontrol.

Additionally illustratively according to this aspect of the invention,passing an ultrasound beam having a frequency greater than 3 MHz,preferably about 5 MHz or more, through the paddle and the compressedbreast tissue includes passing an ultrasound beam having a frequencygreater than 3 MHz, preferably about 5 MHz or more, generated by alinear array ultrasound transducer through the paddle and the compressedbreast tissue in direct contact with the paddle.

Illustratively according to this aspect of the invention, passing anultrasound beam having a frequency greater than 3 MHz, preferably about5 MHz or more, through the paddle and the compressed breast tissueincludes scanning an ultrasound transducer across a surface of thepaddle opposite the surface of the paddle in contact with the compressedbreast.

Further illustratively according to this aspect of the invention,scanning an ultrasound transducer across a surface of the paddleincludes manually scanning an ultrasound transducer across a surface ofthe paddle.

Alternatively illustratively according to this aspect of the invention,scanning an ultrasound transducer across a surface of the paddleincludes automatically scanning an ultrasound transducer across asurface of the paddle.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by referring to the followingdetailed description and accompanying drawings which illustrate theinvention. In the drawings:

FIG. 1 illustrates a perspective view of a detachable,three-dimensional, computer-controlled XYZ mechanical scanner with anultrasonic imaging unit and an x-ray mammography system, all accordingto the present invention.

FIG. 2A illustrates a cranio-caudal view of an embodiment of the presentinvention;

FIG. 2B illustrates a medio-lateral view of an embodiment of the presentinvention;

FIG. 2C illustrates a head-on view of an embodiment of the presentinvention;

FIG. 3 illustrates a perspective view of the mechanical scanner with acompression paddle inserted;

FIG. 4 illustrates connection of the mechanical scanner to a paddleassembly;

FIG. 5 illustrates a plastic paddle deflected by a compressed breast;

FIG. 6A and FIG. 6B illustrate details of a Z-axis positioner;

FIG. 7 illustrates an exploded view of a Z-axis positioner and probeassembly;

FIG. 8 illustrates a linear stepper motor;

FIG. 9 illustrates three-dimensional interpolation in volumetricconstruction;

FIG. 10A, FIG. 10B, and FIG. 10C illustrate perpendicular plane views ofa breast;

FIG. 10D illustrates a view of the intersection of perpendicular viewsof a breast;

FIG. 11 illustrates a computer display of orthogonal plane views; and,

FIG. 12 illustrates a volume-rendering view.

DETAILED DESCRIPTIONS OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 illustrates a combined ultrasound and x-ray imaging systemaccording to the present invention, as applied to multi-modal diagnosticbreast imaging. The illustrated embodiment adapts medically-accepted,FDA-approved, ultrasound linear array imaging systems, x-ray mammographyunits, and compression paddles to realize a diagnostic imaging modalityfor detection and assessment of breast cancer and other breastabnormalities. The modality combines two-dimensional x-ray imaging withthree-dimensional ultrasound imaging, with the breast immobilized in thesame configuration for both. Ultrasound and x-ray views of abnormalitiesare thus spatially registered, simplifying interpretation of breastmasses.

A three-dimensional mechanical scanner 20 carries an x-ray mammographycompression paddle assembly 22, which is, in turn, inserted into anx-ray mammography unit 24 such as, for example, a Lorad x-raymammography machine. A computer control system 26 drives the motionsystems of mechanical scanner 20, using a motion controller 28 via amotion controller cable 30, senses their positions, acquirestwo-dimensional ultrasound image data generated by a linear array probe32 connected by a probe cable 34 to an ultrasound unit 36 such as, forexample, Acoustic Imaging, Inc.'s Model 500S B-Mode Scanner, and acomputer 38 constructs three-dimensional ultrasound views of thecompressed breast 40 of a patient. The three-dimensional images aredisplayed on the computer display 42. Normal two-dimensional ultrasound(B-mode) images are displayed on a B-mode image display 44. These B-modeimages are captured by an image capture device 46. The ultrasound viewsare in spatial registration with a standard x-ray image generated byx-ray mammography unit 24. The x-ray image is produced by electronicsdriving an x-ray tube 48 causing x-rays to be transmitted throughcompressed breast 40 to be received by a suitable x-ray image detector50 within a detector assembly 52. The detector assembly 52 includes alower compression plate 54, or surface, supporting the underside ofcompressed breast 40.

X-ray mammography unit 24 generally comprises a vertical x-ray supportcolumn 56 with a C-arm 58 movably attached, which, in turn, containsx-ray tube 48, a movable paddle mount block 60, and detector assembly52. The mechanical scanner 20 is connected to compression paddleassembly 22, which is plugged into paddle mount block 60. Typically,C-arm 58 can be rotated in a plane parallel to the patient's chest walland positioned vertically so patient's breast 40 can be inserted betweencompression paddle assembly 22 and lower compression plate 54 over arange of angular rotations. Force applied by movable paddle mount block60 compresses the breast 40 under compression paddle assembly 22. Therelationship of the overall imaging system with respect to the patient,or the views that can be realized, includes the standard mammographyviews: cranio-caudal, medio-lateral, and angles in between. Someadditional ultrasound images are also possible.

FIG. 1 illustrates the orientation for a cranio-caudal (head-to-foot)radiographic view. In this view, both the through-transmission andechographic volume ultrasound image can be obtained. Rotating the C-arm58 by 90 degrees counter clockwise on the support column 56, in a planeparallel to the chest wall, the operator can orient the breast 40 for amedio-lateral or latero-medial view (middle-to-side, or side-to-middle).In these views, as in the cranio-caudal view, x-ray and two-dimensionalultrasound images can be obtained in registration. Though x-raymammography is a well-accepted imaging modality for breast cancerdetection, it has several shortcomings. First of all, only athrough-transmission image related to integrated tissue density isobtained. Overlying diagnostic features are summed together, resultingin the possibility that important information is blurred, summed, andoverlaid so it cannot be detected in the x-ray image. A furthershortcoming is that the breast is imaged only up to the chest wall, butthere may be abnormalities further that are not recorded on the x-rayfilm. The present invention provides an additional imaging viewparticularly appropriate for this latter situation. If the paddle mountblock 60 is translated far enough to provide enough room for thepatient's body to fit between compression paddle assembly 22 anddetector assembly 52, then the patient can stand between these, facingthe paddle, when the overall system is rotated into the head-on view. Inthis orientation, the patient can either lean against the paddle or,under motor control, the paddle can contact the breast while the XYZscanner 20 performs a scan. The resulting three-dimensional ultrasoundimage then visualizes features from the nipple into the chest wall tobetter examine regions that are inaccessible to standard mammography.This is the “head-on” position. The illustrated embodiment is capable ofproducing, among others, the scans illustrated in FIG. 2A, FIG. 2B, andFIG. 2C namely, cranio-caudal view 62, illustrated in FIG. 2A,medio-lateral view 64 illustrated in FIG. 2B, and head-on view 66illustrated in FIG. 2C.

Generally, as illustrated in FIGS. 1 and 3, the system comprises athree-dimensional mechanical positioning system for scanning, undercomputer 38 control, an ultrasonic linear array probe 32 across animmobilized breast 40 to yield a three-dimensional image registered to aspatial XYZ coordinate frame 68 such as the frame illustrated in FIG. 3.Such coordinate frames establish direction references oriented asillustrated in FIG. 3 for discussion, for mathematical convenience, andby convention, but the origins of such frames clearly can be fixed atarbitrary points in three-dimensional space. Referring further to FIG.3, mechanical scanner 20 positions linear array probe 32 using an X-axisactuator 70, Y-axis actuators 72-1 and 72-2, and a Z-axis positioner 74.Y-axis actuators 72-1 and 72-2, comprising left Y-axis actuator 72-1 andright Y-axis actuator 72-2, are attached in parallel to support bar 78,forming a rigid frame for overall support. Typically, left Y-axisactuator 72-1 and right Y-axis actuator 72-2 are identical, except fororientation, and are operated in parallel, while the X-axis actuator 70may be the same as Y-axis actuators 72-1 and 72-2 with the possibleexception of its length. The X-axis actuator 70 is mounted across Y-axisactuators 72-1 and 72-2, forming XY positioner 76. This combinationprovides planar positioning for the attached Z-axis positioner 74carrying probe assembly 80. The probe assembly 80 integrates lineararray probe 32, a probe jacket 82, and a probe mount plate 84. Theultrasonic linear array probe 32, mounted in probe assembly 80, isattached to, and is positioned vertically by Z-axis positioner 74 tocomplete a three-dimensional XYZ scanning mechanism. The X-axis actuator70 is mounted to left Y-axis actuator 72-1 and right Y-axis actuator72-2 by left XY plate 86-1 and right XY plate 86-2 respectively. TheZ-axis positioner 74 is mounted to the X-axis actuator 70 by an L-shapedXZ plate 88.

Referring now to FIG. 4, compression paddle assembly 22 is fittedbetween Y-axis actuators 72-1 and 72-2, aligned to support bar 78 by apaddle bracket guide pin 90 and secured to support bar 78 by a captivescrew 92. The captive screw 92 engages a threaded paddle adapter plate94 attached to paddle bracket 96 supporting a paddle yoke 98 with anattached plastic paddle 100. Although FIG. 4 illustrates the paddle 100as being inserted into mechanical scanner 20 with some parts removed, inpractice the paddle 100 can be inserted without any disassembly. It canbe angled into place from below. Paddle 100 can be constructed from, forexample, Lexan® polycarbonate plastic.

One of the aspects of the invention is the development of experiencewith various materials for paddle 100. Specifically, plastics wereinvestigated to determine ones which were appropriate for breastcompression paddles 100. Our research has demonstrated that, whilecertain materials such as plexiglas attenuate ultrasound to a lesserdegree than others, for example, polycarbonates, their other physicalcharacteristics are not as advantageous for breast compression aspolycarbonates. Since both x-ray and ultrasound attenuation aredependent on the thickness of the paddle 100 material, we investigateddecreasing the thickness of the paddles 100 from thicknesses known inthe prior art, for example, 0.12 inch (about 3 mm), while maintainingsafety and ensuring that standard x-ray mammography pressures could beapplied with these thinner materials. Compression paddles 100 wereconstructed from various types of plastics using thicknesses of 0.02inch (about 0.5 mm), 0.04 inch (about 1 mm), 0.06 inch (about 1.5 mm),0.08 inch (about 2 mm), and 0.12 inch (about 3 mm). These paddles 100were evaluated in terms of their ability to permit transfer of highfrequency ultrasound while meeting the required standards for breastcompression paddles. One result of our investigations was the findingthat, with use of current ultrasound breast imaging systems, ultrasoundwith a frequency of 7.5 MHz can be transmitted across a polycarbonatecompression paddle with a thickness of 0.12 inch (about 3 mm). Theseinvestigations also demonstrated that it is possible to design breastcompression paddles from various plastics, such as polycarbonate, whichare thinner than those previously used for x-ray mammography whilepermitting application of standard breast compression pressures. Bydecreasing the thickness of the paddle material, the ultrasoundattenuation is decreased and thus higher ultrasound frequencies can beapplied. Ultrasound frequencies of 13 MHz were used to examine patientsin the standard craniocaudal compressed breast position. Thepolycarbonate paddles 100 designed and fabricated for thoseinvestigations had a thickness of 0.04 inch (about 1 mm). Additionalinvestigations indicated that frequencies as high as 20 MHz can betransmitted through appropriately designed polycarbonate compressionpaddles. These findings are significant. A standard x-ray mammographycompression paddle can be used when 3D ultrasound imaging is performedusing frequencies in the range of, for example, 7.5 MHz to 10 MHz. If itis desired to use higher frequencies, appropriately thinner paddles canbe used.

It is generally recognized in the medical community that x-raymammography is diagnostically beneficial and appropriate for routineexamination of subjects age 40 and older, and generally for any ageadult with evidence of possible pathology, such as palpable breast massor other overt symptom. For subjects under 40 with no symptoms ofpathology, it is recommended that these individuals carry out routineself-examination and, at intervals, undergo a hand-applied breastpalpation examination performed by a physician. X-ray examination is notrecommended as a routine screening method for young, asymptomaticpatients because of the possibly long-term deleterious effects ofexposure to ionizing radiation. In addition, in comparison to resultsachieved with older subjects, x-rays are less effective in detectingpathologies in the normally dense breast tissue of young individuals.

Masses detected by hand breast palpation are generally of the order of 1cm or more in size. At the time of detection, a large sized malignantmass may be accompanied by metastases to other breast or body regions.The presence of (a) large tumor(s) may require a mastectomy rather thansurgical removal of the mass and some surrounding tissue. Life spanfollowing detection of breast cancer is related to the size of the massand its treatment at the time of detection. For young subjects, there isa current need for development of non-ionizing breast examinationinstruments which can improve the detection of small breast masses orother subtle indications of the presence of breast cancers. It ismedically accepted that ultrasound imaging is non-injurious and,compared to x-ray imaging, is more effective in imaging the dense breasttissue of young subjects.

With the exception of Japanese physicians, ultrasound imaging is notgenerally accepted as a breast screening method for either young orolder subjects. The variable results obtained at different medicalfacilities using current ultrasound imaging techniques to examine thewhole breast constitute the primary basis for this lack of acceptance.Detection of a small mass in an unknown breast area by application of anultrasound transducer in contact with a freely movable unencumberedbreast is critically dependent on the technical training of theindividual carrying out the examination. Specific knowledge is requiredon basic mechanisms involved in the interaction of ultrasound with softtissue and an awareness of possible false imaging data associated withan inadequate examination technique. In the United States, technologistsgenerally carry out the ultrasound breast examinations. A solution tothis long-term problem is to design an ultrasound examination systemwhich is machine-based, automatic and capable of improving the detectionof small breast masses in asymptomatic subjects by the use of higherultrasound frequencies than are applied in the prior art. Thecompression paddle knowledge developed during the course of thisinvestigation provides an automated high-frequency ultrasound imagingsystem for breast examination of young, asymptomatic patients. In thedesign of this instrumentation system, the subject can either bestanding or sitting and a compression paddle capable of transmittinghigh frequency ultrasound contacts the entire anterior surface of thebreast, the head-on position. Under motor control, the breast iscompressed by the paddle. This motorized compression decreases the depthof the breast tissue which must be traversed by the ultrasound, thuspermitting the use of higher ultrasound frequencies with their attendanthigher resolution capability. A high frequency linear array ultrasoundtransducer in direct contact with the paddle, using an appropriatecoupling medium as necessary, scans all areas of the breast by means ofautomated linear translation. Patient examinations have been conductedsuccessfully in this head-on orientation using the paddles of bothBennett and Lorad x-ray mammography systems to apply the pressure tocompress the breast. In these examinations, the ultrasound transducerwas scanned manually. It is contemplated that the ultrasound transducerwould also be rotated automatically around the nipple-areolar region toimage the ductal structure, a region where breast cancer may initiallybe evident.

The availability of such high frequency, automated ultrasoundexamination systems is of particular importance for improving detectionof pathologies in the breasts of asymptomatic young subjects. Theavailability of high frequency ultrasound not only increases resolution,but also improves detection of microcalcifications, a significantindicator of possible breast cancer. In the prior art, x-ray mammographyis the best technique for detecting microcalcifications. However, aspreviously noted, x-ray mammography is not the preferred modality forasymptomatic young patients, and so the higher resolution offered byhigher frequency ultrasound offers another mechanism for the detectionof microcalcifications in asymptomatic young patients.

Ultrasound scanning in the head-on position using anultrasound-transparent compression paddle offers numerous advantagesincluding very rapid whole breast imaging suitable for breast screening.Additionally, the patient is in a reasonably comfortable standing orsitting position. Using relatively higher ultrasound frequencies,resolution is sufficiently high to detect microcalcifications previouslyonly detectable by x-ray imaging. Use of the compression paddle toreduce the thickness of the tissue the ultrasound must traverse permitsthe use of these relatively higher frequencies. Automated pressureapplication to the breast in the head-on position can help differentiatethe characteristics of cystic and solid masses. A head-on automatedmachine can be applied both to produce a two-dimensional B-mode scan,and to produce the 3D images of the method and apparatus which form apart of this invention.

Referring to FIG. 5, linear array probe 32 is scanned across plasticpaddle 100 by the XY positioner 76, and is permitted to follow itspossibly curved contour. The plastic paddle 100 may be deflected in the+Z direction during use by compressed breast 40. Deflection as detectedby Z-axis position encoder 102 serves as an input to software executedby the computer 38 (See FIG. 1) for use in three-dimensional imageconstruction. The Z-axis position encoder 102 including encoder linkage104, linear encoder sensor 106, and linear encoder graticule 108,monitors the Z-position of probe assembly (See FIG. 3).

In FIG. 5 it will be noted that probe 32-to-paddle 100 couplant 110 andpaddle 100-to-breast 40 couplant 112 are used during scanning to couplethe ultrasonic waves acoustically to the breast 40 tissue. In order forultrasonic waves to propagate into the breast 40, be reflected by tissueinterfaces within the breast 40, and be received by the probe 32, it isnecessary to use a coupling gel or other coupling material on the topsurface of the paddle 100 (probe 32-to-paddle 100 couplant 110). Acouplant is also required between the underside of the paddle 100 andthe breast 40 (paddle 100-to-breast 40 couplant 112). As illustrated inFIG. 5, compression of the breast 40 generally results in deflection ofthe paddle to form a curved contour. The Z-axis assembly presses theprobe 32 against the top surface of the paddle 100, while the x-ray unitvertical axis presses the paddle 100 to the breast 40. The Z-axispositioner 74 thus presses the probe 32 into the layer of couplingmaterial 110 to maintain acoustic wave coupling and effectivepropagation of waves.

In FIGS. 6A and 6B, limit switches 114, X-axis lower limit switch 116-1,X-axis upper limit switch 116-2, Y-axis lower limit switch 118-1 andY-axis upper limit switch 118-2, are monitored by a motion controller 28(FIG. 1) to determine automatically the maximum X-Y scan extent ofinserted plastic paddle 100. Therefore, various sized standard paddles100, after modifications for mounting, can thus be effectively employedfor various-sized breasts 40, and their size can be detectedautomatically.

Further detailing the Z-axis functions in FIGS. 6A and 6B, there areseveral ways to maintain probe 32 contact. In the illustratedembodiment, this is accomplished using a spring-loaded, floating mountblock on the Z-axis to which is attached the probe mount assembly.Referring to FIG. 6A, Z-axis positioner 74 includes a dovetail base 120,wherein a suspended block 122 and a contact force block 124 can move inthe Z direction. A Z-axis shaft 126 passes through both blocks. Z-axisshaft 126 is threaded over the travel limits, including throughout therange of the contact force block 124 and extending above and belowcontact force block 124 to yield about 0.5 in. of vertical travel.Z-axis shaft 126's threads engage contact force block 124 to apply forcevia upper suspension springs 128 to suspended block 122, which issuspended from below by lower suspension springs 130. The arrangementprovides for adjustment of the probe 32-to-paddle 100 contact force viaa contact force adjuster 132.

In FIG. 7, a probe mount plate 84 carries an encoder linkage 104 to movelinear encoder sensor 106 along linear encoder graticule 108 to trackthe vertical (Z) position of the probe 32 via an electrical connectionbetween a Z-axis position encoder 102 and motion controller 28 (FIG. 1).Unlike previous two-dimensional scan systems of the general typesdescribed in, for example, U.S. Pat. Nos. 5,474,072; 5,479,927;5,640,956; 5,664,573; and, 5,938,613, Z-axis positioner 74 permits theprobe 32 to “float” or ride along the contour of an upwardly curvedplastic paddle 100 as deflected by compressed breast 40 (FIG. 5). Sincethe Z position is sensed as the scan proceeds, the vertical positions oftwo-dimensional images, as generated by ultrasound unit 36, are knownand can be used to perform spatially accurate, three-dimensional imageconstruction. If the probe 32 is fixed in its Z position, as it is in,for example, U.S. Pat. Nos. 5,474,072; 5,479,927; 5,640,956; 5,664,573;and, 5,938,613, then the flexible plastic paddle 100 will be pusheddownward against the breast 40 as images are acquired, distorting thesoft tissue as it passes, thus losing the registration of adjacent imageplanes with respect to each other in the Z direction. Subsequentconstruction (combination, merging, interpolation) of these misalignedplanes into a three-dimensional image will result in inaccurateconstruction of breast 40 features. A typical ultrasound linear arraysystem operating at 7.5 MHz has a resolution of about one to twowavelengths (about 0.2 mm to about 0.4 mm) in the B-mode plane. Thus, ifthe paddle 100 is deflected upwardly, curved, or bent by, on the orderof 0.2 mm to 0.4 mm, the inherent resolution provided by a prior artultrasound unit may be lost, thereby compromising the clinicalacceptability of the whole approach. Errors introduced by ignoring acurved paddle would be relatively larger at higher frequencies. Thinnerpaddles are desirable because they permit higher ultrasonic frequenciesto be used, resulting in higher resolution. However, curvature of thepaddles is more pronounced as the paddles become thinner. The encoded Zaxis permits this curvature of thinner paddles to be accommodated.

The present system is designed to accept linear array probes 32 from avariety of manufacturers to realize the goal of a versatile adapter forthree-dimensional imaging. As best illustrated in FIG. 7, a mechanicaladapter probe jacket 82 adapts a given probe 32 for mounting to a probemount plate 84. The probe jacket 82 is a split plastic block including afront block 134 and a rear block 136 forming a clamp around the probe 32handle, as secured by screws. A probe jacket aperture 138 in the splitblock is formed, for example, by machining, to fit the probe handle.This permits probes 32 that are designed for hand-held operation to beused by the three-dimensional scanner, thus extending the utility ofstandard linear array scanners. The present invention can be adapted tomost ultrasound units, so that it can be used as an “add on” enhancementto existing ultrasound systems, to extend the imaging capability of suchsystems to three dimensions. The probe jacket 82, carrying linear arrayprobe 32, is guided onto probe mount plate 84 by probe mount plate guidepins 140 and secured by a probe mount plate set screw 142 tightenedagainst a probe latching pin 144. Merely loosening probe mount plate setscrew 142 permits removal of the combined probe jacket 82 and probe 32.The probe 32 can be removed, for example, to eliminate it from the x-rayfield of view when small compression paddles 100 are used for spotimaging. Several probes 32, perhaps with different center frequenciesand sizes, are typically provided with standard ultrasound breastimages. The probe assembly mount design is meant to make it convenientto change probes 32 rapidly while maintaining accurate positioning andprobe 32 orientation.

The computer control system 26 illustrated in FIG. 1 is interfaced tothe overall XYZ assembly by wiring 30 for programmed motion in X and Ydirections and sensing of the XYZ position of linear array probe 32, andby interface 31 for acquisition, construction and display of spatiallyregistered ultrasound and x-ray images. The computer display 42 isprovided for user operation of the system, via a graphical userinterface, for monitoring, controlling, and displaying the position ofvarious positional assemblies, initiating diagnostic scans, recallingprevious scans, displaying two and three-dimensional images, and storingresults.

In the illustrated embodiment, the X and Y actuators are linear steppermotors of the general type available from, for example, NorthernMagnetic, Inc., 25026 Ana Drive, Santa Clara, Calif. 91355. A linearstepper motor 146, as illustrated in FIG. 8, includes a platen 148 and aforcer 150 with a motor electrical connector 152. A linear electricmotor can be thought of as a rotary electric motor that has been cutalong the radial plane and unrolled. The resultant motor is capable ofproducing a linear thrust. Although perhaps not as common as rotarymotors, linear motors offer several distinct advantages. First, theplatens 148 can be used to form part of the frame (support) of thescanner, thereby resulting in a more compact design while reducingoverall complexity. In the illustrated embodiment, this permits themechanical scanner 20 to remain attached to the x-ray mammography unit24 while x-ray imaging is performed without interfering with imageformation. This eliminates the need for complicated swing-away or hingedmechanisms to move the mechanical scanner 20 out of the x-ray field ofview. The patient can remain in the same position for both the x-ray andthe ultrasound examinations, permitting the operator to register thex-ray and ultrasound images spatially, thereby facilitating theinterpretation of breast 40 abnormalities. Linear motors are directdrive mechanisms, thereby eliminating backlash inherent with leadscrews, ball screws, belt drives, rack and pinion mechanisms, and thelike. In the illustrated embodiment, two-way, or raster, scanning can beperformed without an encoder, while still maintaining high positionaccuracy without the need for backlash compensation. A more robust,rapid, and simple system results. Linear motors offer furtheradvantages, including improved reliability, a wide range of velocities,smoothness, accuracy, stiffness, and increased overall life expectancy.A linear motor suitable for the present application is available fromNorthern Magnetic as Model 1302, which produces 5 pounds of force. Alarger motor or counterweight may be necessary for smooth operation whenthe scanner is oriented with the X axis vertical.

In operation, the illustrated embodiment performs multi-modal x-ray andthree-dimensional ultrasound imaging of a breast 40 under compression. Atypical patient examination is performed as follows. A suitablecompression paddle assembly 22 is chosen according to the size of breast40 to be imaged, is installed into mechanical scanner 20 (FIG. 3) andsecured as illustrated in FIG. 4.

The scan adapter with mounted paddle 100 is plugged into x-raymammography unit 24 using the standard left paddle assembly mount pin156-1 and right paddle assembly mount pin 156-2. The resulting patientexamination configuration presented to the medical practitioner is verysimilar to the standard x-ray mammography setup, except that mechanicalscanner 20 surrounds compression paddle assembly 22 and carries lineararray probe 32 above paddle 100. An interface cable carrying power andcontrol signals is connected from mechanical scanner 20 to motioncontroller 28. Video output and synchronizing signals of ultrasound unit36 in any suitable format, for example, NTSC, are connected to imagecapture device 46 via, for example, coaxial, video cabling. The imagecapture device 46 is typically a circuit card, for example, a DataTranslation, Inc., Model 1352 capture card, plugged into the bus ofcomputer control system 26. The computer control system 26 can be, forexample, a personal computer (PC), such as, for example, an IBM AptivaModel X computer, running a Microsoft Windows operating system, forexample, Windows 98 or NT.

Once the overall mechanical adapter, x-ray unit, ultrasound unit, andcompression paddle configuration is installed and interconnected, thepatient examination can proceed. It will be understood that such setup,once completed, need not be repeated for subsequent patients. Theillustrated embodiment can be left in place (installed, set up) withoutinterfering with standard x-ray mammography procedures. This isimportant, since accepted medical practice and FDA-approved systems mustnot be compromised for the illustrated embodiment to be practical andefficacious.

The patient is prepared as if for a normal x-ray mammographyexamination, except that paddle 100-to-breast 40 couplant 112 is appliedto the breast 40 prior to immobilizing the breast 40 under the x-raycompression paddle. The couplant 112 must not interfere with diagnosticfeatures in the x-ray image. Since an x-ray mammogram displays theintegrated density of the object through which it is transmitted, paddle100-to-breast 40 couplant 112 that is too thick, or too dense, such thatit, or its boundary, overlays (appears in) the breast 40 image features,would be unacceptable. Examples of suitable paddle 100-to-breast 40couplant 112 include, for example, mineral oil, Jojoba oil, very thinlayers of standard coupling gels, such as Aquasonic 100 coupling gelavailable from Parker Laboratories, Orange, N.J., and water. Compressionof the breast 40 and configuration of the patient with respect to thex-ray unit are performed as for a normal x-ray examination.

The x-ray mammogram is obtained in the standard manner. During x-rayimaging, the XY positioner 76 carrying Z-axis positioner 74 and probeassembly 80 is moved away from the patent's chest wall to the back right(or left) corner of the XY scan extent. If necessary, the probe 32 withits probe jacket 82 is removed from mechanical scanner 20 so that nointerference with the incident x-ray beam occurs.

Three-dimensional ultrasound imaging is performed after the x-ray. Theprobe 32-to-paddle 100 couplant 110 is applied after the x-ray image isobtained, and therefore can have no effect on the x-ray image. Suitableprobe 32-to-paddle 100 couplants 110 include, for example, shallow waterbaths, coupling gel of any thickness, a layer of oil, cream, or otherspreadable material affording ultrasonic wave coupling between probe 32and paddle 100. The thickness or density of probe 32-to-paddle 100couplant 110 is not as critical as for the paddle 100-to-breast 40couplant 112 since, by the time it is needed, the x-ray image hasalready been obtained. If another x-ray is needed after probe32-to-paddle 100 couplant 110 has been applied, the couplant can cleanedfrom the paddle.

Ultrasonic scanning is performed after applying probe 32-to-paddle 100couplant 110. The probe assembly 80 is mounted on the scanner 20, if notalready present. Generally, the operator manually moves the probe 32around over the paddle 100 and visualizes the breast 40 as in a normalultrasound exam. When the current to the linear motors is turned off,the probe 32 can be freely moved by hand, and the breast 40 can beexamined using standard ultrasonic breast imaging protocols. Thetwo-dimensional (B-mode) image, as generated and displayed by ultrasoundunit 36, is examined as manual scanning proceeds. When a region ofinterest is identified, the practitioner leaves the probe 32 at an XYZposition in that vicinity and initiates an automated scan.

Whether or not manual scanning is performed, an automatedthree-dimensional scan is initiated via a software graphical userinterface residing on computer 38 (see FIG. 1). Referring to the XYZcoordinate frame 68 shown in FIG. 3, the linear array probe 32 ispositioned in X and Y to a desired start position. The linear arrayprobe 32 is swept in the +X direction while the video output ofultrasound unit 36 is digitized at a rate of N frames per second, where,for example, N=30. The X-axis actuator 70 is rapidly accelerated fromrest (within 1 mm of travel), and the scan or sweep then proceeds at aconstant velocity. Scan velocity is set via motion controller 28 tosynchronize probe 32 position with image capture, such that images aredigitized at multiples of a selected interval size along X, for example,0.125 mm. The image sequence for each sweep in X is stored in computer38 (See FIG. 1) memory, or on disk. The XY positioner 76 then shifts theprobe 32 in the −Y direction, and another scan is performed along X asneeded to cover the Y-extent of the desired breast region. The shift in−Y direction is typically set so that digitized, captured adjacentB-mode image planes will overlap from sweep to sweep. The overall XYscan motion and image capture can be a “raster” scan pattern, where thefirst sweep is in +X direction; shift in the −Y direction; the secondsweep is in −X, back to the starting X; and so on, until the region iscovered. Alternatively, image acquisition can be made to occur only on+X sweeps, where the probe 32 is returned to X start position beforeeach shift in −Y.

If lead screw, rack and pinion, or other drive systems exhibitingbacklash are used to implement the mechanical system, unidirectionalscanning might provide greater accuracy. Use of linear stepper motorsessentially eliminates backlash, so that a rapid and efficient rasterscan can be reliably performed without the need for complex backlashcorrection schemes. The resulting minimization of total scan (imageacquisition) time minimizes the time the breast 40 must be held undercompression, and thus reduces patient discomfort. Total scan timesexperienced in a system constructed and operated according to thepresent invention were in the range of about 1 minute long.

Once individual two-dimensional, overlapping image sequences areobtained, they are constructed by specialized software residing oncomputer 38 (See FIG. 1) into a three-dimensional volumetric image.Construction is illustrated in FIG. 9, and proceeds as follows. Imagescaptured to the computer 38 from the commercial ultrasound linear arrayscanner are used to form a volume image. The volume image is representedin computer software as a three-dimensional array of volume samplevalues, while two-dimensional images are represented by two-dimensionalarrays of image values. In order to form a volume image from thecaptured images, one must perform interpolation from where the imagesamples fall in space to where one wants to locate the regular array ofvolume image values. This is the well-known mathematical problem ofinterpolation. In general, as is illustrated in FIG. 9, a volume valueis calculated as the weighted sum of neighboring image values. Theweights are chosen such that if the volume sample point falls exactly onone of the image points, the volume value is set equal to the value ofthe corresponding image value. Otherwise, the volume value will becomputed to depend upon the contributing image values in inverseproportion to the distances of the particular image samples from thevolume sample, or some other function of the distance. Interpolation inthree dimensions is a direct and well-known extension of methodsdesigned for two dimensions in image processing as described for examplein: Udupa, J. K., et al., 3D IMAGING IN MEDICINE, CRC Press, 1991;Foley, J. D., et al., COMPUTER GRAPHICS: PRINCIPLES AND PRACTICE, Seconded., Addison-Wesley, 1990; Foley, J. D., et al., FUNDAMENTALS OFINTERACTIVE COMPUTER GRAPHICS, Addison-Wesley, 1982; and, Gonzalez, R.C., et al., DIGITAL IMAGE PROCESSING, Addison-Wesley, 1992. It should berecalled that the scanner of the present invention acquires images inmultiple sweeps across the paddle to cover the region of interest. Thelocation of each image plane is known from the motion controller and theencoded Z axis position. Therefore, the spatial information is known forall planes for use in formulas to form a volume image. The volume imageitself can be set to encompass all planes, or a subregion for azoomed-in view.

The volumetric image is displayed in several formats on computer display42 for comparison to the spatially-registered x-ray image. Theseinclude, but are not limited to, intersecting perpendicular ororthogonal planes of the breast (See FIG. 1A, FIG. 10B, FIG. 10C, FIG.10D, and FIG. 11), and translucent volume-rendered views (See FIG. 12).Once a volume image is obtained, it can be displayed in a variety offormats on the computer display 42. One format which applicants'research has shown to be an effective approach is to show theintersection of the three individual perpendicular planes of a breast asillustrated in FIG. 10A, FIG. 10B, and FIG. 10C, and/or the intersectionof the individual perpendicular planes as illustrated in FIG. 11D. Auser interface implemented in computer software can employ a mouse tointeractively select particular planes and to play them back in a movieloop which pages through slices in one particular direction. Many otherformats can be used, as is well known in the three dimensionalvisualization art. Computer constructed intersecting perpendicular, ororthogonal plane views are illustrated in FIG. 11. Volume-renderedviews, as illustrated in FIG. 12, are also generated with the aid of thecomputer 38.

Operation of systems constructed according to the present inventionresults in x-ray and ultrasound images that are in spatial registration.The medical practitioner can view the x-ray along with top-view planesor projected views of the volumetric ultrasound. Visually and mentally,the practitioner can then fuse, merge, or visualize the imagingmodalities as qualitatively registered, and compare the same abnormalityappearing in both. However, additional reference information is requiredfor the computer 38 to quantitatively register image points, that is,for the operator to know that a point (X1, Y1) in an x-ray image is inthe same spatial location in world coordinates as the projected orsliced ultrasound view at (X2, Y2), where (X1, Y1) and (X2, Y2) arerelative to a reference coordinate frame. Software in a systemconstructed according to the present invention is designed to permiteasy integration of this information, and to permit display ofregistered x-ray and ultrasound images side-by-side on the computer 38monitor. It should thus be understood that the present invention canaccept and be enhanced to perform quantitative registration given aparticular physical registration target arrangement. In other words,specific registration approaches can be incorporated into the presentinvention. Prior art systems of the types illustrated and described inU.S. Pat. Nos. 5,474,072; 5,479,927; 5,640,956; 5,664,573; 5,938,613;5,603,326; 5,776,062; and, 5,840,022 are described as producingregistered images. However, the prior art does not develop specificcomputable referencing methods to support the claim that quantitativeregistration can be practically performed in multi-modal breast imagingby the prior art systems.

In another embodiment, the three-dimensional scanner is mounted to anindependent support device, post, or articulated arm so that it canoperate either in conjunction with x-ray mammography, other imagingsystems, or independently, for three-dimensional ultrasound-onlyimaging. Many variations of supports, such as, for example, commerciallyavailable C-arms designed for medical imaging, can be employed tosupport the mechanical scanner 20. Any support which can position thescan assembly in the desired orientations can be used.

The stand-alone version can be used, for example, with an x-ray machine,or alone as an ultrasound-only imaging device, or with other suitablemedical imaging modalities where a system constructed according to thepresent invention can be mounted to, or situated to interact with suchimaging modality. The compression paddle 100 can be replaced by aflexible paddle or a flexible membrane of, for example, a rubber-like orelastomeric material. Because the Z position of the scanning probe 32 isencoded, the flexure of the paddle or membrane is sensed by the computer38 and used to form accurate volumetric constructions. The apparatus andmethod of scanning illustrated and described herein can significantlydecrease patient time and permit relatively precise correlation betweenx-ray mammograms and whole breast ultrasound examination. The evaluationof the entire scanning volume of a mass can significantly improveaccuracy in diagnosis. It is believed that the close correlation ofdigital x-ray mammographic images with whole breast ultrasound madepossible by the method and apparatus illustrated and described hereinwill lead to improved accuracy in detecting malignant breast masses.

Another technique for examination of the young breast was to replace thecompression paddle and film holder of a standard x-ray mammographymachine with ultrasound transparent paddles of the general types andconfigurations described above. The presence of two opposing ultrasoundtransparent paddles provided a number of advantages. For example, whenimaging the breast in the craniocaudal position, it is also possible toultrasonically scan the breast in the caudocranial position withoutmaking any changes to the C-arm positions. All that is necessary is torun the linear array transducer along the surface of the inferiorpaddle. Caudocranial scanning is of particular advantage for the largebreasted patient with a mass that is located deep behind the anteriorwall of the breast held in the craniocaudal position. In this situation,that mass is located close to the inferior aspect of the breast surfaceand can thus be readily visualized by a transducer scanning over thesurface of the inferior paddle. An additional advantage of thecaudocranial position is that the inferior aspect of the breast in thisposition is extremely flat. This permits direct, that is, perpendicular,entrance of the ultrasound beam. This normal angle of incidence has anadvantage in terms of non-divergence of the beam on entry. An additionaladvantage is that the ultrasound beam entering via the flat inferiorbreast surface can image all of the tissues in its path including thenipple-areola region. This is one solution to the problem ofdifficulties encountered when scanning across the anteriorly locatedcompression paddle in the craniocaudal position, namely, the failure toimage the nipple-areola because of lack of contact between thenipple-areola and the compression paddle.

In the oblique positions or any of the lateral positions, the doubleultrasound transmitting paddle permits imaging from whicheverorientation is most advantageous. For example, in the lateral positionthe ultrasound beam may enter the breast form either the medial orlateral side. This ability to ultrasonically image the breast in variousorientations improves correlation between ultrasound and x-raymammography images. Again, this approach can be implemented in either 2Dor 3D imaging.

1. A method for examining a breast of a subject, the method comprisingthe steps of: providing an apparatus including a compression paddleconstructed from a plastic material, the paddle comprising a firstsurface against which the breast is compressed and a second surfaceopposite the first surface, an ultrasound probe for generatingultrasound image data of the breast through the material of thecompression paddle and in spatial registration with the compressionpaddle, a motion control system for movement of the probe in relation tothe compression paddle and for sensing the probe's position, the motioncontrol system comprising a Z-axis positioner for movement of the probein a dimension perpendicular to the compression paddle, and a computerfor generating an ultrasound image from the ultrasound image data andfrom information regarding the spatial registration; compressing thebreast with the paddle causing the breast to be in contact with thefirst surface of the paddle; and generating ultrasound image data of thecompressed breast through the plastic material of the compression paddleutilizing the motion control system for movement of the probe inrelation to the compression paddle, wherein the Z-axis positioner isutilized for maintaining contact of the probe with the second surface ofthe paddle.
 2. The method of claim 1, further comprising the steps of:providing an x-ray tube for generating x-ray data; and generating x-raydata of the compressed breast.
 3. The method of claim 2, furthercomprising the steps of: providing a computer operably connected to theultrasound probe and to the x-ray tube; and receiving the ultrasoundimage data and the x-ray data with the computer.
 4. The method of claim1, wherein the step of generating the ultrasound image data furtherrequires engagement of the X-axis actuator and Y-axis actuator togenerate ultrasound image data comprising two dimensions.
 5. The methodof claim 1, wherein the step of generating the ultrasound image datafurther requires engagement of the X-axis actuator and Y-axis actuatorto generate ultrasound image data comprising three dimensions.
 6. Themethod of claim 1, wherein the step of utilizing the motion controlsystem further requires that the motion control system utilize theZ-axis positioner for maintaining contact of the probe with the secondsurface of the paddle by movement in a direct perpendicular to theX-axis and Y-axis actuators.
 7. The method of claim 6, wherein the stepof generating the ultrasound image data further requires engagement ofthe first and second axis control to generate ultrasound image datacomprising two dimensions.
 8. The method of claim 7, further comprisingthe steps of: adjusting the third axis control; and repeating the stepof generating the ultrasound image data to generate ultrasound imagedata comprising three dimensions.
 9. The method of claim 8, furthercomprising the steps of: repeating the steps of adjusting the third axiscontrol; and generating the ultrasound image data to generate ultrasoundimage data comprising three dimensions until substantially all of thebreast is represented by such ultrasound image data.
 10. The method ofclaim 6, wherein the step of providing a motion control system furtherrequires that the first, second, and third axis controls beautomatically actuated.
 11. The method of claim 1, wherein the step ofproviding a motion control system further requires that at least one ofthe first axis control or third axis control is manually actuated. 12.The method of claim 1, wherein the step of providing a motion controlsystem further requires that at least one of the X-axis actuator, Y-axisactuator, or Z-axis positioner is automatically actuated.
 13. The methodof claim 1, wherein the paddle has a thickness between about 0.02 inch(about 0.5 mm) to about 0.12 inch (about 3 mm), the method furthercomprising the step of: providing a transducer operably connected to theprobe for generating an ultrasound beam from which the ultrasound imagedata is generated, the ultrasound beam having a frequency greater thanabout 5 MHz.
 14. A method for generating an ultrasound image of acompressed breast of a subject, the method comprising the steps of:providing an apparatus including a compression paddle constructed from aplastic material, the paddle comprising a first surface against whichthe breast is compressed and a second surface opposite the firstsurface, an ultrasound probe for generating ultrasound image data of thebreast through the material of the compression paddle and in spatialregistration with the compression paddle, a motion control system formovement of the probe in relation to the compression paddle and forsensing the probe's position, the motion control system including aY-axis actuator, an X-axis actuator, and a Z-axis positioner formovement of the probe in three dimensions; compressing the breast withthe paddle causing the breast to be in contact with the first surface ofthe paddle; and scanning the breast with the ultrasound probe throughthe material of the compression paddle utilizing the motion controlsystem for movement of the probe in relation to the compression paddle,wherein the Z-axis positioner is utilized for maintaining contact of theprobe with the second surface of the paddle.
 15. The method of claim 14,further comprising the step of: adjusting one of the first-axis controlor the second-axis control.
 16. The method of claim 15, furthercomprising the step of: repeating the step of scanning the breast.
 17. Amethod for generating an ultrasound image and an x-ray image of acompressed breast of a subject, the method comprising the steps of:providing an apparatus comprising an x-ray mammography unit forgenerating the x-ray image, the x-ray mammography unit including acompression paddle, a vertical x-ray support column having a movable armsupporting an x-ray tube, a compression paddle mount block movable alongthe vertical x-ray support column, the compression paddle connected tothe compression paddle mount block and being both radiolucent andsonolucent, and the paddle comprising a first surface against which thebreast is compressed and a second surface opposite the first surface, anultrasound scanner connected to the compression paddle mount block, thescanner comprising an ultrasound probe for generating ultrasound imagedata in spatial registration with the compression paddle, a motioncontrol system for movement of the probe in spatial relation to thecompression paddle and for sensing the probe's position, the motioncontrol system including a Z-axis-controller for controlling themovement of the probe toward contact with the compression paddle, and anx-ray detector assembly including an x-ray image detector and a lowersupport surface for supporting a compressed breast; compressing thebreast with the paddle causing the breast to be in contact with thefirst surface of the paddle; scanning the compressed breast with theultrasound scanner utilizing the motion control system for movement ofthe probe in relation to the compression paddle, wherein the Z-axiscontroller is utilized for maintaining contact of the probe with thesecond surface of the paddle; and scanning the compressed breast withthe x-ray tube.
 18. The method of claim 17, further comprising the stepof: adjusting the motion control system.
 19. The method of claim 18,further comprising the step of: repeating the step of scanning thecompressed breast with the ultrasound scanner.
 20. The method of claim17, wherein the step of providing an apparatus further requires that theapparatus include a computer operably connected to the scanner and thex-ray mammography unit for receipt of the ultrasound image data and thex-ray image.
 21. A method for examining a breast of a subject, themethod comprising the steps of: providing an apparatus including acompression paddle suitable for contacting a surface of the breast, thecompression paddle being both radiolucent and sonolucent, and of athickness between about 0.02 inch (about 0.5 mm) to about 0.12 inch(about 3 mm), and constructed of polycarbonate plastic, a device forapplying pressure to the surface of the breast with the compressionpaddle to compress the breast to reduce the thickness of the breasttissue, a transducer for generating an ultrasound beam having afrequency greater than about 5 MHz for transmission through thecompression paddle and the compressed breast tissue and for receivingechoes from the compressed breast tissue through the compression paddle,and a device for converting the received echoes into breast imagingdata; compressing the breast; and scanning the breast with thetransducer.